It is a known procedure to provide modern aircraft with constructions to enhance the crash safety which, in case the aircraft crashes or falls, reduce the risk of injury to passengers, flight crew and pilots is reduced and increase their chance of survival. To this end, such constructions are designed in such a way that, in case of a crash, they are deformed in a controlled manner, a process in which they absorb the highest possible amount of impact energy, and sufficiently absorb and limit impact loads. This, in turn, serves to reduce negative accelerations or load factors that occur in a crash, thus also reducing the loads that act on the above-mentioned occupants of the aircraft.
During a crash, especially rotary-wing aircraft such as, for example, helicopters, exhibit typical impact angles and impact velocities that can be predicted with quite good precision within a statistical framework. Moreover, the impact velocities are relatively low as a rule; they are often lower than those encountered in most vehicular accidents. For this reason, crash safety concepts are particularly promising for use with rotary-wing aircraft. Known airframe constructions that enhance the crash safety of helicopters employ shock-absorbing floor structures that are deformed in a controlled manner in case of a hard impact, so that this material deformation absorbs a large amount of the impact energy that is generated.
An airframe such as, for instance, the airframe of a rotary-wing aircraft or of a helicopter, has to be constructed so as to be not only high in strength but concurrently also very light in weight. These general requirements for lightweight construction mean that the airframe and its support structure can only be dimensioned for certain load factors or limit loads. The crash safety of the airframe is configured for these limit loads. When the airframe is designed, it has to be additionally taken into account that the area of the airframe that extends above the floor structure—which normally has a frame-like support structure with, for example, frame-shaped elements—encloses a cockpit or cabin area that is provided as a compartment for the aircraft occupants. Consequently, this frame-like support structure has to be designed so as to be especially stable and rigid in order to create a survival space for the aircraft occupants in case of a crash and also to prevent heavy components such as, for example, gears and/or rotors located above the airframe, from destroying this survival space.
If additional loads in the form of mobile pieces of equipment (for example, military backpacks for airborne troops) or permanently installed equipment (e.g. medical equipment, measuring devices, etc.) are attached to the support structure of the airframe, in case of a crash, these pieces of equipment generate large additional loads that are introduced into the support structure as high load peaks. This, however, means that the support structure, which also has to absorb the pulse-like loads stemming from the masses that are inherent to the aircraft in case of a crash, can become overloaded and fail. In other words, the crash safety of the airframe would no longer be ensured and the aircraft occupants would be greatly at risk.
In order to prevent this, the entire airframe would have to be designed to be so strong and stable right from the start that it can reliably absorb all of the additional loads and every load peak that occur. This, however, would translate into an extremely heavy construction, which runs fundamentally counter to the requirements for lightweight construction in aircraft. Especially in case of already existing airframe constructions, it would also be possible to retrofit the airframe structure with extra reinforcements. This measure, however, would likewise considerably increase the overall weight of the airframe. Moreover, such retrofitted reinforcements can only be realized with extremely high levels of technical expertise and skilled labor.